It is known in the prior art to control the speed of a vehicle propulsion direct current series motor by a chopper apparatus including a thyristor switch device in series with the motor, as shown in U.S. Pat. Nos. 3,769,566 and 4,284,930. Speed control of the motor is provided by varying the width of voltage pulses supplied to the motor such that the resulting average power applied to the motor establishes the operational speed thereof. A commutation circuit including a charge capacitor has been used to provide a biased voltage across a conducting thyristor device for commutating the conduction of that thyristor device. A propulsion motor is operative in a motoring mode when the passenger vehicle is being propelled or accelerated along a track and in a brake mode when the vehicle is being stopped or decelerated.
As described in a published article in the Westinghouse Engineer for March 1973 at pp. 34-41 the average voltage applied to the motor armature is controlled by adjusting the ratio of chopper OFF-time to the chopper ON-time with the resulting average motor armature current determining the motor torque for moving the vehicle along the track.
In the motoring or power mode of operation the motors of a vehicle are connected in relation to a direct current voltage source such that current is supplied through a chopper when the chopper is ON and through the motors to ground. When the chopper is turned OFF, the energy stored in the motor reactor and the inductance of the motor field maintains current flow in the motor circuit through a freewheeling diode. In the brake or deceleration mode of operation the motors in the prior art were reconnected by mechanical reverser switches with the motor operative as self-excited generators to provide dynamic or regenerative braking of the vehicle. With the chopper ON the motor current increases and with the chopper off the current is forced into the power supply through the free-wheeling diode by the motor reactor.